Combine harvesters have been developed for efficient harvesting of a wide range of crops. To this end a variety of machine settings can be changed and adjusted. These include the setting of the louvres of the upper and lower sieves. The opening between these louvres has to be adapted to the grain type in order to obtain an efficient separation of grain on the one hand and chaff and straw on the other hand. Especially the opening of the lower sieve has to be adjusted to a precise sieve setting in order to meet optimum grain cleaning requirements. Sieves enabling such adjustments have already been described in U.S. Pat. No. 2,253,296, issued Aug. 19, 1941 in the name of William A. Holtzman.
Usually the opening of the upper sieve is verified by the insertion of calibrated means such as gauges between the louvres at the rear of the sieve. The surface of the upper sieve mostly is readily accessible for such measurement. The lower sieve however is not as easily accessible because of its hidden position under the upper sieve. Access to the louvres of the lower sieve can further be hindered by extensions connected to the rear of the upper sieve, such as a rake or a sensor plate for a grain loss indicator. The use of calibrated means for the verification of this opening thus is rendered impractical.
Several methods have been thought of to by-pass this problem. On sieves with an adjustment handle which is locked into a toothed segment, it is a common practice to count the number of teeth that lay between a leftmost position, in which the louvres are closed, and the actual position of the handle. But this is not an accurate measurement of the sieve opening, because the leftmost position does not correspond to the same tooth on every sieve and the opening determined by one tooth does not necessarily correspond to a step of, e.g., exactly one millimeter. Further inaccuracies occur because of the tolerances on the dimensions of the intermediate adjustment links and handles, because of the play of these components relative to each other and because of the possible deformation thereof during operation.
The German Utility Model No. 69 05,754, issued in the name of Kodel and Bohm, Gmbh., shows a graduated scale applied to an adjustment member of the sieve opening. Other types of sieve opening indicators on adjustment means can be found in U.S. Pat. No. 3,472,378, issued Oct. 14, 1969 in the name of Amos O. Payne. These indicators may give an opening read-out in millimeters or fractions of an inch, but the zero position which corresponds to fully closed sieve louvres, is not indicated. Neither do these systems cope with the inaccuracies caused by the tolerances, play and deformation of the adjustment means intermediate the adjustment member and the louvres.